Most industrial applications that utilise a working fluid to provide cooling or lubrication for example, require fluid filtration devices which serve to maintain the quality of the fluid by removing solid contaminant material. If filter elements were not employed the resulting heavily contaminated fluid would no longer flow freely resulting in excessive machine wear and ultimately machine failure. Moreover, certain types of contaminant material, in particular metal particles, can act to catalyse unwanted chemical reactions within the machinery whereby the working fluid effectively becomes increasingly corrosive.
A number of categories of fluid filtration devices have been proposed capable of filtering mediums such as oil, water or other natural or synthetic liquids.
A first type of fluid filter that is widely used in vehicles, aircraft and industrial machinery removes microparticles from the fluid via a permeable membrane typically being conventional filter paper. The fluid passes through the paper medium whereby the microparticles are deposited on the paper as the fluid passes through it. However, such filters are disadvantages as the replaceable filter cartridges contribute to maintenance costs and present waste disposal problems.
Magnet filters have emerged as a suitable alternative to paper filters and function to separate particulate impurities via the magnetic flux generated by a magnetic core within the filter.
Conventional magnetic filters typically have an outer case surrounding an inner magnetic core. The fluid to be filtered, flows between this outer case and the magnetic core such that contaminant material is attracted and deposited on the outer surface of the core.
GB 1129516 discloses a magnetic filter having an inner magnetic core comprising a series of coaxial discs of a soft ferromagnetic material. Each disc is positioned between coaxial discs of permanent magnets forming a cylindrical magnetic core. So as to prevent separated contaminant material from being removed from the magnetic core by the passing fluid and to extend the depth of field created by the magnets, the annular ferromagnetic spacers have a greater radius than the permanent magnets so as to form a ribbed column. As the contaminated fluid flows over the magnetic core, the contaminant is attracted by the magnets and trapped between the ribs formed by the annular ferromagnetic extensions which serve to prevent the passing fluid removing the contaminant from the magnetic core given the relatively weak magnetic field employed.
EP 0873774 similarly discloses a paperless magnetic filter comprising a column of coaxial magnetic discs. Positioned between each magnetic disc are supporting structures and yokes which are of larger radius than the magnets creating the ribbed structure. The magnets are arranged such that neighboring magnets in the column have repelling polarity (N to N, S to S), creating a perturbed magnetic field. When the fluid flows through the magnetic field turbulence is induced and the contaminant particles are more readily separated from the fluid with regard to a non-repelling magnetic arrangement.
Whilst being beneficial over more traditional paper filters, by obviating the requirement for a replacement filter medium, a number of problems exist with conventional magnetic filters. Due to the construction of the magnetic core from disc magnets, the magnetic field created is relatively weak necessitating the annular ferromagnetic extensions which both extend the magnetic field and prevent separated contaminant from being washed-off the outer surface of the central core. Such filters are limited in that they can separate only relatively small volumes of contaminant before the filter becomes contaminant-saturated and requires cleaning. Cleaning such filters requires the fluid to be drained and the filter dismantled causing expensive operational interruptions. Additionally, as the contaminant is trapped within the profiled ribs of the column, cleaning typically comprises spray or jet washing so as to sufficiently remove the contaminant. This elaborate cleaning process in turn introduces additional operational delays.
There is therefore a need for a magnetic filter that addresses these problems.